This invention relates to an under-reamer system for boreholes, especially for under-reaming horizontal and extended reach boreholes.
When, in petroleum extraction, the diameter of a section of a borehole is to be increased an under-reamer is used. The under-reamers which are found on the market today have reaming members, i.e., cutters or crushing means, secured to outwardly pivotal arms which are hinged to a main body. This construction is unfavourable with regard to strength. The arms provide little space for the cutters, and moreover it is difficult to position the accompanying nozzles for cleaning and cooling the cutters in an expedient manner. The activation of today's under-reamers is based on pressure drop across a nozzle or on the use of a ball which is sent with the mud flow from the drill floor. The possibility of putting in several under-reamers in a series, in such a way that worn-out under-reamers can be deactivated and new ones activated, is thus limited. This means to say that worn-out equipment must be hauled out of the borehole in order to be replaced by new equipment. Moreover, today's under-reamers do not have any stabilization on the upper side, which makes them poorly suited to extended reach and horizontal boreholes. Such stabilization is necessary in order to take up the lateral load which is brought upon the equipment by its own weight and that of the drill string.